U.S. President Donald Trump on Monday promised "big sanctions" against Turkey for its invasion into northern Syria against Kurdish fighters, but again defended his withdrawal of American troops who had been fighting alongside the Kurds against Islamic State terrorists.

"Do people really think we should go to war with NATO Member Turkey?" Trump said on Twitter. "Never ending wars will end! The same people who got us into the Middle East mess are the people who most want to stay there!"

Two key congressional leaders, House Speaker Nancy Pelosi, leader of the Democratic-controlled House of Representatives, and Senator Lindsey Graham, a Trump Republican ally, agreed in a phone conversation on the need for quick action condemning Turkey's attacks on the Kurdish fighters.

Pelosi called for a congressional resolution "to overturn the president's dangerous decision in Syria immediately." Graham said he would work with Democrats to "draft sanctions and move quickly."

Senate Majority Leader Mitch McConnell said he was "gravely concerned" about the Turkish offensive, contending that it will jeopardize "years of hard-won progress" in destroying Islamic State's self-declared caliphate in northern Syria. McConnell said he would hold talks with other lawmakers and Trump administration officials this week on how "to avoid a strategic calamity" in the region.

Trump pledged Sunday to work with congressional leaders, including opposition Democrats, to impose the economic sanctions on Ankara.

The U.S. leader contended that the Kurdish fighters "may be releasing some" Islamic State prisoners in an effort "to get us involved." But Trump said they could be "easily recaptured by Turkey or European Nations from where many came."

Syrian forces entered a town near the Turkish border Monday, a day after reaching an agreement with Syrian Kurds to move into the region in an attempt to counter the Turkish onslaught.

Syria's state-run SANA news agency reported Monday's troop movement in Tal Tamr, about 20 kilometers from the border, saying it was done to "confront the Turkish aggression" and was welcomed by the people there.

The extraordinary deal between the Kurds, Syria, and Russia - Syria's main ally - comes four days after Turkish forces moved against the Kurds in northern Syria after Trump withdrew nearly all U.S. forces.

Turkey regards the Kurdish Syrian Democratic Forces as terrorists aligned with separatists inside Turkey, although Western countries have viewed them as a key ally in the fight against Islamic State.

A Kurdish statement said, "In order to prevent and block this assault, agreement has been reached with the Syrian government whose duty is to protect the borders and Syrian sovereignty, for the Syrian army to enter and deploy along the length of the Syrian-Turkish border."

Syrian Kurds say they feel totally forsaken by the United States after fighting side-by-side with U.S. forces. They also believe much of the Arab world and the U.N. Security Council are ignoring them.

"We had to find a solution... now you see what has happened to us and they don't want to support us," the defense minister of the Kurdish city of Kobani, Ismat Sheikh Hassan, said Sunday. "Again, we should not trust anybody. We should rely on ourselves. Every Kurd should carry a gun and prepare himself."

With the United States moving to withdraw all of its remaining troops from northern Syria, Trump said Sunday, "Very smart not to be involved in the intense fighting along the Turkish Border, for a change."

"The Kurds and Turkey have been fighting for many years," he wrote on Twitter. "Turkey considers the PKK the worst terrorists of all. Others may want to come in and fight for one side or the other. Let them!"

U.S. Defense Secretary Mark Esper told Fox News Sunday there will be a "deliberate withdrawal" of the last 1,000 U.S. troops from northern Syria.

"We pushed back on Turkey to not do this operation," Esper said. But he said Turkey was "fully committed to do this no matter what we did."

"This is part of the terrible situation that Turkey has put us in," Esper said, adding that the U.S. "didn't sign up to fight Turkey, a long-time ally, on behalf of the Kurds."

French President Emmanuel Macron and German Chancellor Angela Merkel said at a joint news conference that Turkey is risking an "unbearable humanitarian situation."

The World Health Organization said Monday it is "gravely concerned about the humanitarian health situation in northeast Syria."

It said the military operations have already displaced 200,000 people since last week, and that there are fewer health workers able to provide services due to displacement and health providers suspending operations.